1. Field of the Invention
This invention relates to the calcination and desulfurization of delayed petroleum coke by programmed contact with sulfur-bearing gases at elevated temperatures for metallurgical and chemical applications.
The high sulfur content of many dalayed cokes renders them unsuitable for important commercial uses. This has limited use of such cokes, which are relatively inexpensive, readily available refinery by-products, as sources of raw materials and energy for metallurgical and chemical applications. In such applications, the sulfur in the coke poses problems of end product quality, manufacturing productivity and pollution control. Current trends in crude oil supply portend still higher sulfur levels in the future.
Carbon products such as anodes for aluminum production and electrodes for ferrous metallurgy are conventionally made from calcined delayed coke. Delayed coke is obtained from a variety of feedstocks (reduced crude, vacuum resid, thermal tar and decant oil) by fractionating these materials and by further cracking of the heavy fraction in coke drums to yield vapor and coke. The major type of delayed coke, "sponge coke", is a porous, crystalline material which, after calcining to remove volatiles and to refine the structure, is a suitable ingredient for carbon products.
Another form of delayed coke is "needle coke", which can be produced in delayed cokers. It has superior physical and chemical properties which make it suitable for more demanding uses such as electrodes for ferrous metallurgy. Needle coke is even more crystalline than sponge coke and provides even greater hardness and strength.
The sulfur in such delayed cokes is a function of the sulfur in the feedstock from which the cokes are made. High sulfur in the coke is not substantially removed by conventional calcining and can carry through to the end carbon product, causing structural deficiencies and other undesirable qualities in the end carbon product. This invention provides a process that substantially desulfurizes delayed coke so as to permit accommodation of higher sulfur feedstocks in the production of delayed cokes.
Cokes produced by the fluid coke process are unacceptable for anode production without special treatment because of their physical and chemical properties. In a process distinctly different from delayed coking, fluid coking converts heavy, low-grade oil into a coke which has an onion-skin, relatively amorphous structure which does not provide the required hardness, strength, bonding and handling characteristics required in anode manufacture. It does not graphitize properly during processing. Further, its relatively high coefficient of thermal expansion and its low electrical conductivity adversely affect carbon product quality. Fluid coking tends to produce a product of unacceptably high metals content by nature of the process. Accordingly, the majority of fluid coke is burned as a boiler fuel by the producing refiner and is reserved for the dirtiest feedstockes. Its poorer qualities for carbon products have led to the development of processes to maximize the liquid and gaseous products from such coke and to minimize the solids, e.g., by gasifying the coke. In the few instances where special technology and further grinding are applied to fluid coke for use in carbon products, it must be blended with delayed coke to minimize adverse impacts on end product quality.
2. Description of the Prior Art
Desulfurizing methods disclosed in the prior art involve extended treatment periods (up to several hours) at elevated temperatures or involve intimate contact with liquid desulfurizing agents. Such prior art methods may require excessive energy, capital investment, material costs, and additional steps in product purification and waste stream treatment. Those desulfurization methods which expose the coke to inert gas do not achieve maximum desulfurization.
This invention provides modification of conventional calcining facilities, some of which may be co-located at refineries where the delayed coke and the sulfur gases are produced, to permit programmed introduction of sulfur gases (such as refinery sour gas, hydrogen sulfide, mercaptans, or other sources of active sulfur) as desulfurizing agents, producing by a gas-solid reaction a low sulfur (less than one percent by weight) end product otherwise similar to conventionally calcined delayed petroleum coke. Recovery and recycle of effluent gases would be by conventional means.
Prior publications and patents describe efforts to use heat-carrying inert gas to desulfurize cokes. As described in the Oil and Gas Journal, Jan. 22, 1979, pg. 64-68, a thermal process developed by C-E Lummus and Institute Mexicano del Petroleo involves from three to nine hours of treatment at elevated temperature. Desulfurized coke yield, not addressed in the publication, may be adversely affected by prolonged exposure at high temperature.
U.S. Pat. No. 4,160,814 discloses a thermal process with data showing extensive desulfurization with nitrogen as the inert medium but there is no suggestion that other gases may be used.
U.S. Pat. No. 3,009,781 deals only with fluid coke which, as described earlier, is generally unacceptable for the uses intended for the invention. The patent presents a two-stage process in which the first stage is intended to raise the thermal conductivity of the bed to permit electrothermic production of carbon disulfide and to reduce the sulfur in the fluid coke, and in which the second stage involved passing a stream of gas through the bed. The gas is selected from the group consisting of nitrogen, carbon monoxide, hydrogen, mixtures of carbon monoxide, hydrogen and nitrogen, and hydrogen sulfide. However, U.S. Pat. No. 3,009,781 pat. does not correctly distinguish between inert gases, and active gases, such as carbon monoxide which attacks the carbon and hydrogen sulfide which decomposes to form an active sulfur agent. In addition, temperature control disclosed in U.S. Pat. No. 3,009,781 pat. is insufficient to separate thermal effects from other effects. Moreover, no account is taken for the attack on carbon alone with concomitant coke loss versus other possible chemistry. U.S. Pat. No. 3,009,781 pat. also incorrectly implies a continuous improvement in sulfur removal with increased temperature and holding time.
U.S. Pat. No. 4,011,303 is the first to disclose the chemical effect of gaseous, active sulfur to remove sulfur from the coke. Without reference to heating prior to reaction, U.S. Pat. No. 4,011,303 pat. discloses the use of elemental sulfur vapor diluted with nitrogen as the agent in a one-step process in which the elemental sulfur combines with carbon-sulfur groups in the coke (the desired reaction) and with carbon alone in the coke ( undesired because it results in carbon loss with little desulfurization).
When sulfur is vaporized, as in U.S. Pat. No. 4,011,303, various species of sulfur are generated (e.g., S.sub.8, S.sub.6, S.sub.2). However, active sulfur species can be generated by other means. For example, it is known that hydrogen sulfide (H.sub.2 S) decomposes at elevated temperature to form gaseous hydrogen and sulfur. Carbon monoxide and sulfur dioxide react to form carbon dioxide and gaseous sulfur. Carbonyl sulfide in the presence of water produces sulfur. Thus, active sulfur may be generated for use in desulfurization by decomposition of sulfur gases and by reaction of gases containing sulfur. This chemistry is incorporated in the subject invention.